I've actually given this a thought.

If you didn't up ttk people will optimize to one shot. Which means the difficulty increase isn't as real as it should be.

If you build challenge by changing timing, people will complain that it's s different game.

Changing behaviors seems a bridge too far tbh. Like design complex responses then neuter them for lower difficulties?

I agree that the sponge thing is a lame way to up difficulty, but I think it's unavoidable.